Belarus' Victoria Azarenka returns the ball to Germany's Mona Barthel during the …

Maria Sharapova needed more than three hours to complete a marathon three-set comeback against US Open winner Sam Stosur to reach Saturday's semi-finals at the Stuttgart WTA clay-court tournament.

With Sharapova, Victoria Azarenka, Petra Kvitova and Agnieszka Radwanska all in Saturday's semi-finals, this is the first time since Wimbledon in 2009 that the world's top four players have reached the last four at a WTA tournament.

Having won the first set tie-breaker, Stosur served for the match at 5-4 in the second only for Sharapova to hit back to claim a 6-7 (5/7), 7-6 (7/4), 7-5 victory after three hours, one minute on court.

The world number two hit 11 aces, compared to Stosur's seven, and as the quarter-final finished at just before 2300 local time, the Russian said recovery was her main concern.

"She has a great serve and she served as well as she ever has against me," said Sharapova, who faces third-seed Kvitova in Saturday's semi-final after the Wimbledon champion's 6-4, 6-4 win over Germany's Angelique Kerber.

"It was pretty difficult to play against her, so I am very happy with the win. It's not over until the last point has been played and you've shaken hands.

"She had her chances to win, so I am just happy to have reached the last four. I have a second chance now and I intend to use it."

Stosur was in philosophical mood after a high-quality match when the only break point came in the third set.

"It sucks to lose, but when you play as well as I did tonight, you can't be too hard on yourself," she said.

"It was a very good match, we both played well and there wasn't much in it. Towards the end of the third set, she started serving very well and put me under pressure to get the only break point.

"The last game was the cherry on top for her winning at 40-0 with some very good serving."

In the other semi-final, top seed and world number one Azarenka will play fourth-seed Radwanska of Poland.

All of Radwanska's four losses this year have come at the hands of Azarenka and having also reached the semi-finals here last year, the Polish number one said it takes a special effort to beat the world's top female player.

"You have to be at the top of your game and play some great tennis to beat her," admitted Radwanska.

Azarenka needed nearly three hours to see off German wildcard Mona Barthel 6-4, 6-7 (3/7), 7-5 as her big-hitting opponent smashed down 13 aces.

Having already beaten ex-world number one Ana Ivanovic and seventh-seed Marion Bartoli on the way to the quarters, Barthel put up another battling performance to take the match to three sets.

This was Azarenka's fourth win over the 21-year-old German this year, but she had to work for it.

"She keeps producing some great tennis and she showed again today why she is a tough one to beat," said Azarenka.

"Her serve is definitely up there with the best, after more than two-and-a-half hours she was still blasting down aces. I have been saying for a while she is one to watch."

Radwanska booked her place in the semis by beating French Open champion Li Na of China 3-6, 6-2, 6-3.

The Chinese struggled to cope as Radwanska raised her game with a series of drop shots and strong returns in the second set before carrying her improved form into the third.

With Roland Garros to start on May 27, Radwanska said she was enjoying finding her feet on clay courts as she bids to win her first Grand Slam title.

"I am enjoying my tennis and just trying everything at the moment, which probably worked against me in the first set," said the 23-year-old, who claimed her ninth tour title at Miami recently.

"I was more calm in the second set and I am just happy I could turn things around. I just hope this is my year, so I can take the next step."

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